Users Guide

Table Of Contents
Layer-2 Multicast Features 927
What Is L2 Multicast Traffic?
L3 IP multicast traffic is traffic that is destined to a host group. Host groups
are identified by class D IPv4 addresses, which range from 224.0.1.0 to
239.255.255.255, or by FF0x:: or FF3x:: IPv6 addresses. In contrast to L3
multicast traffic, Layer-2 multicast traffic is identified by the MAC address,
i.e., the range 01:00:5e:00:00:00 to 01:00:5e:7f:ff:ff:ff for IPv4 multicast traffic
or 33:33:xx:xx:xx:xx for IPv6 multicast traffic.
When a packet with a broadcast or multicast destination MAC address is
received, the switch will flood a copy into each of the remaining network
segments in accordance with the IEEE MAC Bridge standard. Eventually, the
packet is made accessible to all nodes connected to the network.
This approach works well for broadcast packets that are intended to be seen or
processed by all connected nodes. In the case of multicast packets, however,
this approach could lead to less efficient use of network bandwidth,
particularly when the packet is intended for only a small number of nodes.
Packets will be flooded into network segments where no node has any interest
in receiving the packet.
What Is IGMP Snooping?
IGMP snooping allows the switch to snoop on IGMP exchanges between
hosts and multicast routers and perform multicast forwarding within a VLAN.
The IGMP snooping feature complies with RFC 4541. When a switch “sees”
an IGMP report from a host for a given multicast address, the switch adds the
host's interface/VLAN to the L2 multicast group forwarding table and floods
the report to all ports in the VLAN. When the switch sees a leave message for
the group, it removes the host interface/VLAN from the L2 multicast group
forwarding table.
IGMP snooping learns about multicast routers by listening for the following
messages:
An IGMP query packet.
PIMv1 (IGMP type 0x14) packets with destination IP address 224.0.0.13.
DVMRP (IGMP type 0x13) packets with destination IP address 224.0.0.4.
PIMv2 (IP protocol type 0x67) packets with destination IP address
224.0.0.13.